Civil Engineering Reference
In-Depth Information
Figure 11.1
Hydration and dehydration reactions in the system CaO-SO
3
-H
2
O.
DH, dihydrate (CaSO
4
.2H
2
O); HH, hemihydrate (CaSO
4.
½H
2
O); A
=anhydrite (CaSO
4
).
The rate of hydration differs in the various calcium sulfates: whereas calcium sulfate
hemihydrate and anhydrite-III hydrate rapidly, the hydration of anhydrite-II progresses
very slowly.
T
he possible hydration and dehydration reactions in the system CaO-SO
3
-H
2
O are
summarized in Fig. 11.1.
11.2
GYPSUM PLASTERS
“Gypsum plasters” is the general designation for inorganic binders that contain
anhydrous calcium sulfate or calcium sulfate hemihydrate as their main or sole
constituent. The setting and hardening of these binders is associated with their hydration
to calcium sulfate dihydrate. The water requirements and reactivity vary greatly in
different members of this group of binders, and this in turn affects their rate of strength
development and ultimate strength. Gypsum plasters are not suitable for loadbearing
applications, owing to the high creep of the hardened material under load. Also, calcium
sulfate, and thus also all binders yielding calcium sulfate dihydrate as the product of the
hydration process, exhibit a limited yet distinct solubility in water, and thus should not be
